The correct answer is D. Thoughtful.
A contemplative person is one who is given to thought or reflection. They may be deep in thought, or they may be simply taking the time to appreciate their surroundings. Contemplative people are often seen as being calm and peaceful, as they are not easily distracted by the hustle and bustle of everyday life.
Morbid means having or showing an interest in unpleasant subjects, especially death and disease. Serious means having or showing a great deal of thought or care. Calm means free from agitation or excitement.
While all of these words could be used to describe a contemplative person, thoughtful is the most accurate. A contemplative person is someone who is given to thought, and thoughtfulness is a key component of contemplation.